The present invention relates to a power assisted steering apparatus which is applicable to an automotive vehicular steering system.
Generally, in order to facilitate a steering operation through a steering wheel with less force, various types of power assisted steering apparatuses in which supply and exhaust of a working oil (hydraulic) into or from a (hydraulic) actuator installed in the vehicular steering system are carried out to achieve a steering assisted power have been used.
A Japanese Patent Application First Publication No. Showa 61-85272 exemplifies one of the previously proposed power assisted steering apparatuses which is provided with an actuator constituted by a regulating valve and a power cylinder, a hydraulic circuit interposed between an oil pump driven by means of an electric motor and the actuator, an accumulator installed in the hydraulic circuit, a pressure sensor arranged for detecting a hydraulic pressure in the hydraulic circuit, a controller arranged for receiving a detection signal from the pressure sensor and controlling a drive or stop of the motor according to the received detection signal.
In the disclosed vehicular power assisted steering apparatus, when the pressure sensor detects a reduction in the hydraulic pressure in the hydraulic circuit, the working oil is accumulated into the accumulator so that, during an abrupt steering operation, an inconvenience for a discharge oil quantity to become insufficient can be eliminated because of its rise characteristic of the hydraulic pump in a start operation.
However, since the pump for the hydraulic pressure generation is driven and rotated by means of the motor, a driving energy of an engine mounted in the vehicle is not only reduced according to a working efficiency of the motor but also is reduced according to a power generation efficiency of an alternator of the engine so that an energy loss becomes large. In addition, since the motor is controlled according to the detection signal from the pressure sensor for electrically detecting the hydraulic pressure in the hydraulic circuit, the pressure sensor and the controller are required so that a whole power assisted steering apparatus becomes expensive.
A Japanese Patent Application First Publication No. Heisei 4-321803 exemplifies another previously proposed power assisted steering apparatus which is provided with a variable capacity pump, a spring biased in a direction such that the discharge quantity of the variable capacity pump is decreased, a regulator biased in a direction such that the discharge quantity of the variable capacity pump is increased, a hydraulic circuit arranged for introducing the discharge oil of the variable capacity pump into the actuator via a cargo control valve, an accumulator arranged for holding the remaining pressure of the hydraulic circuit up to a predetermined value, an open/closure valve disposed within a pressure introducing passage linked between the hydraulic circuit and regulator, and a cargo switch operatively opening or closing the open/closure valve.
When the actuator is not operated, the open/closure valve is closed to zero the discharge quantity of the working oil from the variable capacity pump and the remaining pressure of the hydraulic circuit is introduced into the regulator which is held up to the predetermined value by means of the accumulator, thus increasing the discharge quantity of the variable capacity pump.
Since, in the latter previously proposed power assisted steering apparatus, the variable capacity pump is always operated and during non-operation of the actuator the discharge quantity of the variable capacity pump is zeroed so as to avoid an energy loss, it is not necessary to install a clutch mechanism so as to halt the drive to the pump during the non-operation of the actuator, in order to avoid the energy loss and the whole apparatus becomes simplified.
In addition, since, in the latter previously proposed power assisted steering apparatus, during the abrupt steering operation, both of the open/closure valve and the cargo control valve are opened so that the remaining pressure in the hydraulic circuit which is held to the predetermined value by means of the accumulator is speedily supplied to the regulator and actuator, thereby the discharge quantity of the variable capacity valve being increased and the power assisted steering operation being achieved.
However, in the latter previously proposed power assisted steering apparatus, the discharge quantity of the variable capacity pump is zeroed during the non-operation of the actuator. If the discharge quantity of the variable capacity valve is zeroed, the working oil is left in an inside of the pump and a suspended state of the working oil is continued. In this state, in addition to a heat generation of a constituent part of the pump due to a frictional slide thereof, a temperature of the working oil is increased so that an earlier deterioration of the working oil may be resulted. A reduced durability in the power assisted steering apparatus is involved.